1. Field of the Invention
The present invention relates generally to a CMOS image sensor, and more particularly to a CMOS image sensor, which can minimize a reflectance of light at an interface between a photodiode and an insulating film, thereby enhancing image sensitivity thereof.
2. Description of the Prior Art
A CMOS image sensor includes a plurality of pixels, each of which consists of several transistors for various purposes and one photodiode. In such a CMOS image sensor, first of all, light transmits the photodiode made of Si while exciting holes formed in the photodiode to generate electrical signals and then the electrical signals are converted into an image.
It is ideal that 100% of light reaches the photodiode, but an area ratio occupied by the photodiode per pixel is reduced to 30% or less as a pixel size becomes smaller and smaller due to the requirement for high picture quality. That is, only 30% or less of actual incident light can enter into the photodiode. For this reason, there is employed a method in which a micro lens is formed above each pixel to focus 90% of light on the photodiode.
Meanwhile, since multilayer metal interconnections are formed above the photodiode and the transistors, light incident through the micro lens passes multilayer insulating films and then arrives at the photodiode. In this course, light does not transmit the insulating films by 100%, but is reflected to a certain extent. The larger a difference between refraction indices of materials is, the more light is reflected. In general, places where reflection occurs most frequently are an interface between a photodiode, a kind of doped silicon, and a SiO2 based insulating film. A reflectance at such an interface amounts to about 15% as shown below in Table 1.
Table 1 shows a reflectance at the interface between the photodiode made of Si and the SiO2 based insulating film used above the photodiode. Here, it can be seen that the photodiode has a refraction index (n) of 3.4 and the insulating film used above the photodiode has a refraction index (n) of 1.5.
TABLE 1MaterialnReflectanceSi3.4SiO21.50.150354019Total reflectance15.04
An alternative proposal to reduce the reflectance to 15% or less employs a structure in which a semi-reflection film of SiN having a refraction index of 2.1 is coated between the photodiode and the insulating film, which results in a reflectance of about 8%. However, even if such a structure is employed, there is still a problem in that the sum of reflectance values occurring at an interface between the micro lens and the insulating film or between every two layers of the multilayer insulating films brings about a loss of at least 10%.